It is a technique for visualizing volumes, in which volume data, obtained by computed tomography (CT) or magnetic resonance imaging (MRI) for example, can be displayed on a screen. Known examples of such visualization techniques are known by the terms “volume rendering technique” (VRT) or “gradient magnitude rendering”. The 3D image data, present as a matrix of scalar values, must be assigned optical properties in these rendering techniques. This is effected by a suitable choice of transfer function, which takes into account optical absorption and emission in the volume to be displayed. This transfer function can also determine which parts of the volume to be displayed are displayed opaquely, semi-transparently or transparently in the image. Furthermore, specific colors can be assigned to individual voxels by way of this transfer function.
When displaying 3D volume data records from tomographic imaging modalities, in particular MRI or CT image data records, using a volume rendering technique, it is possible for structures of interest lying on the inside to be covered by regions, such as bones, lying on the outside. For example, the cranial bones cover the brain structures in MRI images. Although certain regions can be made transparent by manipulating the transfer function, this can hardly be achieved in a satisfactory fashion in the case of MRI images of the head because in the 3D image data the cranial bones have very similar intensity values to the brain structures. It is for this reason that clipping techniques are often used in this case, in which the cranial bones have to be segmented laboriously so that they can subsequently be masked.
A further technique for visualizing 3D image data from tomographic imaging modalities using a rendering technique is disclosed in Ch. Rezk-Salama et al., “Opacity Peeling for Direct Volume Rendering”, in: Computer Graphics Forum (Proc. Eurographics), vol. 25, issue 3, pages 597 to 606, 2006, the entire contents of which are incorporated herein by reference. In the method disclosed therein, a peeling technique is used in which outer, non-transparent layers of the displayed volume can be peeled off or made transparent. The method uses the known ray-casting algorithm, in which each image pixel is calculated by integrating or summing along a ray from the eye of the observer through the volume surrounding the 3D image data. In this case, the sum or integral includes the corresponding transfer function with an emission and/or absorption component. Hence, the transparencies of the individual voxels along the ray are summed. When calculating the individual pixels by starting from the eye of the observer, i.e. in so-called front-to-back composition, a threshold for the optical depth is set. All contributions of the 3D image data to the transparency are set to zero until the threshold value is reached. This makes it possible to peel off or mask an outer layer region of the volume having a constant optical depth which would otherwise cover inner regions. This technique is also referred to as opacity peeling.
However, very thin layers with a high transparency, which remain after the layer has been peeled off, can lead to a distracting pixel flickering of the rendered image data in the utilized pixel shader in this threshold technique.
U.S. Pat. No. 6,532,017 B1 discusses a volume rendering pipeline comprising a single integrated circuit which is intended to ensure cost-effective volume visualization in real time. This document also discloses a ray-casting technique combined with a transfer function for visualizing the 3D image data. The transfer function assigns colors and transparencies to the individual data values in a conventional manner.
US 2007/0236496 A1 describes a graphic art display method for CT images, in which techniques such as duplication, symmetry inversion, contrast inversion, superposition of a number of images or deformation of the image contents, if need be combined with coloring, are applied to obtain a graphic art display.